l‘J2 
GEOLOGY OF THE FOURTH DISTRICT 
Joints or Vertical Cleavage. 
The rocks of this group, where exposed in cliffs or in the banks of ravines, are traversed by 
numerous joints, usually vertical to the planes of stratification, but sometimes slightly oblique, 
and again more or less curved. These will be more fully described in the chapter relating 
to that subject alone. The following sketch exhibits the appearance of a natural section of 
the Moscow shale on the banks of Cayuga lake. 
75 . 
Jointed structure of the Cliffs on Cayuga lake. From a sketch by Mr. E. N. Horsford. 
It will be seen that these joints do not all continue through the part of the mass exposed, 
but the blocks exhibit a series of steps which limit the depth to which these divisional 
planes extend. This structure is very favorable to the rapid abrasion and undermining of 
the cliffs along the lake shore. The water finds access to the slightly opened fissures, which, 
in the colder seasons, are enlarged by the freezing of the same, and in summer the waves 
dash into them, constantly widening the breach. A slight undermining causes a separation 
by the joints in the opposite direction, and the waves in time leave them standing in isolated 
blocks, which are gradually undermined, and fall into the lake. Examples of this kind are 
frequently seen, not only in this group, but in those next succeeding, and more especially the 
Portage group, where they are illustrated. 
Concretions or Septaria .—These bodies, presenting various fantastic forms, are of frequent 
occurrence in this group, though not so abundant as in the shales below and above. Many 
of them, as before noticed, are calcareous concretions, which appear to have aggregated 
around a fossil body or a small nodule of iron pyrites. In the latter case the mass usually con¬ 
tains pyrites, and it is probably owing to the attractive force of this substance that they were 
produced. Fossil shells are frequently found in the centre of these bodies, and the outer 
surface is often covered with them. 
